In general, conventional aqueous ink compositions basically comprise a dyestuff, a polyhydric alcohol or its corresponding ether as a wetting agent and water. In order to achieve good ink jet recording, these conventional ink compositions are required to have various properties. They should have appropriate values in viscosity, surface tension, specific electric conductivity, density and the like. These properties allow adequate droplet formation and control of the jetting directions of the droplets. Further, they should not form any precipitation during storage, in use or at rest over a long period of time. Furthermore, they should not suffer from any significant variation in the physical properties.
An outlet of a nozzle for a recording device has, in general, a diameter of 10 to 60 microns, and any precipitation within the nozzle disturbs ejection of the aqueous droplets of ink from the nozzle. Even if the nozzle is not completely clogged, it is likely to produce solid or viscous material around the outlet of the nozzle, which leads to the depression of the physical property of the ink composition so designed to have that property within a desired range, whereby deterioration of the recording ability as well as the stability or response in jetting will be caused. Still, the printed image as recorded must have not only sufficiently high contrast and clarity but also water resistance, light resistance and the like.
As a cyanic dyestuff (a kind of a blue dyestuff) for the aqueous ink composition, conventionally employed are water soluble direct dyes and acid dyes. Examples of such cyanic dyestuff are C.I. Direct Blue 1, 8, 71, 76, 78, 108, 163, 195, 202 and 236 and C.I. Acid Blue 1, 7, 126, 175 and 234 (cf. Japanese Patent Kokai Publication (unexamined) Nos. 145771/1980, 5772/1982 and 141257/1983).
Since the conventional direct dyes have, however, disadvantages such that they have low solubility in water, sufficient image concentration and contrast are not expected to a satisfactory degree, and they agglomerate and precipitate during storage or in use over a long period of time and are apt to produce the clogging in the nozzle. In addition, they produce a printed image with insufficient water resistance. To overcome these drawbacks as seen in the conventional cyanic dyestuffs, it has been proposed to incorporate in the aqueous composition an additive such as an organic amine or a surfactant as a solubilizer. However, such additive corrodes various parts of the recording device and may invite cloggings in the nozzle due to bubbling of ink composition, whereby the printed image is not sufficiently clear.
On the other hand, the use of the conventional acid dyes may enhance the color tone but produces only an printed image with insufficient qualities, particularly poor water resistance. Therefore, it is necessary to use a special paper for recording as in the case of the direct dyes.